How can I improve bathroom ventilation?

If you have a fan installed already, clean it by removing the cover, taking down and rinsing off the grille, and using a cloth and vacuum to get rid of all the dirt and dust that's accumulated on the fan blades and in the housing. Get into the habit of always running the fan after you shower, or opening a window if you can. If you don’t have a fan, consider investing in one: It’s not a terribly expensive upgrade, and it cuts down on the growth of mold and mildew.